MUSEUM OF COMPARATIVE ZOOLOGY. 391 



which is expelled in the case of the acephalous moUusks. His opinion, 

 although subsequently substantiated, was not based on a satisfactory 

 knowledge of the exact nature and sequence of events in the supposed 

 metamorphosis. 



What relation, if any, exists between the disappearance of the germi- 

 native vesicle and fecundation 1 or, granting the derivation of the polar 

 globules from the germinative vesicle, what is the relation between fe- 

 cundation and the formation of polar globules *? 



Many excellent observers have raised these questions without being 

 able to arrive at a definite conclusion. Thus Loven ('48, p. 535) is un- 

 certain, in the case of Modiolaria, whether the migration of the vesicle to 

 the surface of the yolk and the dissolution of its membrane are processes 

 which pertain to the life of the egg or to the development engendered by 



regard to Dumortier ('37, pp. 10, 11, 15, PI. I.) the case seems much clearer. Six 

 hours after the egg is deposited "on remarque sur le cdte un hile muqueux et 

 diaphane (Fig. 1, K. a) qui est la vesicule de Purkinje." Notwithstanding the erro- 

 neous notion of the ultimate fate of this "hile muqueux," I think there is not only a 

 probability, but almost a certainty, that it is the polar globule. The recent deposit 

 of the egg is in itself, perhaps, enough to make this interpretation probable, especially 

 in connection with the statement that the egg is at this time always totally roimd a.ml 

 opaque ; but there is additional evidence in the further description given by Dumor- 

 tier. In his account of the egg on the second day he says : " Le hile de son cote 

 s'est prolonge et parait forme de deux globules diaphanes, qui ne tardent pas a se 



separer et a se detacher I'un de I'autre (Fig. 2, C. a. h) Toutefois le hile dis- 



paraissant completement le 4^ jour pour ne rei)araitre que le 8^ jour." So far as 1 know, 

 there is only one phenomenon beside the formation of the polar globules to which 

 this description could possibly apply ; viz. the elimination of drops of an entirely 

 transparent fluid previously accumulated between the segmentation spheres. There 

 is little probability that these descriptions are applicable to the latter process. In 

 the first stage the complete rotundity and the opacity of the yolk make it improbable 

 that the first segmentation had been completed ; for (although the first two segmen- 

 tation spheres become mutually flattened so as to give the whole apjiroximately the 

 spherical form again) the accumulation of the liquid would have led the observer to 

 qualify his assertion that the yolk was opaque ; but if the first segmentation had not 

 taken place, then the liilum could not have been due to the accumulated liquid, — at 

 least, I am not aware that any one has observed such accumulations previous to the 

 mutual flattening which follows the first cleavage. In the second stage the descrip- 

 tion and figures (Figs. 2 B, 2 C, 3 J5) of two transparent globules, which do not 

 disappear till the fourth day, can certainly have nothing whatever to do with the 

 elimination of drops of clear fluid, which Dumortier himself saw and subsequently 

 very well described (see p. 15, and description of Fig, 8 B, p. 44) as "une goutelette 

 de liquide qui s'etendit bientot dans I'albumen comme une goutte de lait qui tombe 

 dans I'eau." Since, then, these descriptions do not relate to this elimination of 

 liquid, I think it will be diflficult, if indeed possible, to find any better interpretation 

 than the one which they have for many years enjoyed. 



